1. Field of the Invention
This invention relates in general to shape memory alloys (also called SMA), and more particularly to the fabrication of shape memory alloys in thin film form.
2. Description of the Related Art
Stainless steel remains the material of choice for many fields of medical devices implanted in the human body. For example, stents made of stainless steel are used to prevent restenosis after balloon angioplasty. Titanium-nickel alloy (TiNi), which when annealed so that it crystallizes possesses shape memory properties, has also come into increasing use for stents and other applications. To date, TiNi has been primarily used in tube or sheet form.
Various methods exist to draw tube or roll sheets of shape memory alloys like TiNi. These methods cannot guarantee the perfectly smooth surface finish required for various types of biomedical implants, especially those that will have contact with blood flow. A surface imperfection on a device like a stent can cause an embolism.
In addition to the problem of surface imperfections, material produced by drawing, hot rolling, or cold working techniques cannot match sputter-deposited materials for minimum possible thickness. Nor can such techniques be used to fabricate devices small enough for certain applications. There is a growing impetus for smaller and smoother stents that can be used in very small blood vessels such as those found in the brain to treat aneurisms or filter out blood clots.
Sputter deposited thin film shape memory alloys such as thin film comprised of TiNi overcomes these problems. Such films can be fabricated in a range of thickness from less than 1 xcexcm to 40 xcexcm. SMA material can be made in thin film configurations in accordance the teachings of U.S. Pat. No. 5,061,880 to A. David Johnson et. al., the disclosure of which is incorporated by this reference.
As is well known, an SMA material that has been annealed into a crystalline state undergoes a crystalline phase transformation from martensite to austenite when heated through the material""s phase change transformation temperature. When below that temperature the material can be plastically deformed from a xe2x80x9cmemory shapexe2x80x9d responsive to stress. When the SMA material is heated through the transformation temperature, it forcefully reverts to its memory shape while exerting considerable force.
It is a general object of the invention to provide methods for fabricating shape memory alloys into free standing thin films.
Another object is to provide free standing thin film structures made by such methods.
Another object is to provide methods for fabricating an SMA thin film that is biocompatible, free standing, smooth and uniform in thickness, thereby making it suitable for many medical devices and other applications.